U.S. patent application number 11/529465 was filed with the patent office on 2007-01-25 for exhaust gas-purifying catalyst.
This patent application is currently assigned to CATALER CORPORATION. Invention is credited to Asuka Hori, Nobuhiko Kajita, Keiichi Narita, Yasunori Sato, Ichiro Takahashi, Hirohisa Tanaka.
Application Number | 20070021294 11/529465 |
Document ID | / |
Family ID | 35196781 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070021294 |
Kind Code |
A1 |
Hori; Asuka ; et
al. |
January 25, 2007 |
Exhaust gas-purifying catalyst
Abstract
An exhaust gas-purifying catalyst includes a zeolite and a
perovskite composite oxide containing palladium placed at its B
site.
Inventors: |
Hori; Asuka; (Kakegawa-shi,
JP) ; Narita; Keiichi; (Kakegawa-shi, JP) ;
Sato; Yasunori; (Kakegawa-shi, JP) ; Tanaka;
Hirohisa; (Ikeda-shi, JP) ; Takahashi; Ichiro;
(Ikeda-shi, JP) ; Kajita; Nobuhiko; (Ikeda-shi,
JP) |
Correspondence
Address: |
MORRISON & FOERSTER, LLP
555 WEST FIFTH STREET
SUITE 3500
LOS ANGELES
CA
90013-1024
US
|
Assignee: |
CATALER CORPORATION
Kakegawa-shi
JP
DAIHATSU MOTOR CO., LTD.
Osaka
JP
|
Family ID: |
35196781 |
Appl. No.: |
11/529465 |
Filed: |
September 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP04/04509 |
Mar 30, 2004 |
|
|
|
11529465 |
Sep 27, 2006 |
|
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Current U.S.
Class: |
502/60 ; 502/63;
502/64; 502/65; 502/66; 502/73 |
Current CPC
Class: |
Y02A 50/20 20180101;
B01D 2255/407 20130101; B01J 37/0244 20130101; B01J 23/63 20130101;
B01D 2255/50 20130101; B01J 23/002 20130101; B01J 23/894 20130101;
B01J 2523/00 20130101; B01J 37/0246 20130101; B01D 2255/1023
20130101; Y02A 50/2324 20180101; Y02T 10/12 20130101; Y02T 10/22
20130101; B01J 29/76 20130101; B01D 2255/402 20130101; B01D 53/945
20130101; B01J 2523/00 20130101; B01J 2523/31 20130101; B01J
2523/41 20130101; B01J 2523/824 20130101; B01J 2523/00 20130101;
B01J 2523/3706 20130101; B01J 2523/3712 20130101; B01J 2523/3725
20130101; B01J 2523/48 20130101; B01J 2523/00 20130101; B01J
2523/31 20130101; B01J 2523/41 20130101; B01J 2523/48 20130101;
B01J 2523/822 20130101; B01J 2523/824 20130101; B01J 2523/828
20130101; B01J 2523/00 20130101; B01J 2523/3706 20130101; B01J
2523/824 20130101; B01J 2523/842 20130101; B01J 2523/00 20130101;
B01J 2523/31 20130101; B01J 2523/41 20130101; B01J 2523/48
20130101; B01J 2523/824 20130101 |
Class at
Publication: |
502/060 ;
502/063; 502/064; 502/065; 502/066; 502/073 |
International
Class: |
B01J 29/04 20060101
B01J029/04; B01J 29/06 20060101 B01J029/06; B01J 21/00 20060101
B01J021/00 |
Claims
1. An exhaust gas-purifying catalyst comprising a zeolite and a
perovskite composite oxide having palladium placed at its B
site.
2. The catalyst according to claim 1, wherein the zeolite comprises
.beta.-zeolite.
3. The catalyst according to claim 1, wherein the perovskite
composite oxide is represented by a formula (1):
(A.sup.1).sub.a(A.sup.2).sub.1-a(Pd).sub.b(B').sub.1-bO.sub.3 (1)
where A.sup.1 represents at least one first rare-earth element
selected from the group consisting of rare earth elements taking no
valence other than trivalence, A.sup.2 represents at least one
second rare-earth element selected from the group consisting of
rare-earth elements excluding the rare-earth elements that can have
a valence lower than trivalence, B' represents at least one element
selected from the group consisting of transition elements excluding
cobalt, palladium and rare-earth elements, and aluminum, and a and
b represent atomic proportions of the respective elements, and
0<a.ltoreq.1 and 0<b<1.
4. The catalyst according to claim 1, further comprising a
zirconia-based composite oxide represented by a formula (2):
Zr.sub.1-(x+y+z)Ce.sub.xLa.sub.yLn.sub.zO.sub.2 (2) where Ln
represents at least one element selected from the group consisting
of neodymium, praseodymium and yttrium, and x, y and z represent
atomic proportions of respective elements and satisfy the following
relationship: 0.2<x+y+z.ltoreq.0.6, 0.12.ltoreq.x.ltoreq.0.5,
0.ltoreq.y.ltoreq.0.48, 0.ltoreq.z.ltoreq.0.48, and
0.08<y+z.ltoreq.0.48.
5. The catalyst according to claim 4, wherein the perovskite
composite oxide is carried on the zirconia-based composite
oxide.
6. The catalyst according to claim 1, further comprising a platinum
group noble metal other than palladium.
7. The catalyst according to claim 1, wherein the catalyst
comprises a first layer and a second layer provided thereon, the
first layer contains the zeolite, and the second layer contains the
perovskite structure composite oxide.
8. The catalyst according to claim 7, wherein the zeolite comprises
.beta.-zeolite.
9. The catalyst according to claim 1, wherein the perovskite
composite oxide is represented by a formula (1):
(A.sup.1).sub.a(A.sup.2).sub.1-a(Pd).sub.b(B').sub.1-bO.sub.3 (1)
where A.sup.1 represents at least one first rare-earth element
selected from the group consisting of rare-earth elements taking no
valence other than trivalence, A.sup.2 represents at least one
second rare-earth element selected from rare-earth elements
excluding those rare-earth element that can take a valence lower
than trivalence, B' represents at least one element selected from
the group consisting of transition element excluding cobalt,
palladium and rare-earth elements, and aluminum, a and b are atom
proportions of respective elements, and 0<a.ltoreq.1 and
0<b<1.
10. The catalyst according to claim 1, wherein the second layer
further comprises a zirconia-based composite oxide represented by a
formula (2): Zr.sub.1-(x+y+z)Ce.sub.xLa.sub.yLn.sub.zO.sub.2 (2)
where Ln represents at least one element selected from the group
consisting of neodymium, praseodymium and yttrium, and x, y and z
represent atomic proportions of respective elements and satisfy the
following relationship: 0.2<x+y+z.ltoreq.0.6,
0.12.ltoreq.x.ltoreq.0.5, 0.ltoreq.y.ltoreq.0.48,
0.ltoreq.z.ltoreq.0.48, and 0.08<y+z.ltoreq.0.48.
11. The catalyst according to claim 10, wherein the perovskite
composite oxide is carried on the zirconia-based composite
oxide.
12. The catalyst according to claim 5, further comprising a third
layer, on the second layer, containing a platinum group noble metal
other than palladium.
13. The catalyst according to claim 12, wherein the third layer
further includes a zirconia-based composite oxide represented by
Formula (2): Zr.sub.1-(x+y+z)Ce.sub.xLa.sub.yLn.sub.zO.sub.2 (2)
where Ln represents at least one element selected from the group
consisting of neodymium, praseodymium and yttrium, and x, y and z
represent atomic proportions of respective elements and satisfy the
following relationship: 0.2<x+y+z.ltoreq.0.6,
0.12.ltoreq.x.ltoreq.0.5, 0.ltoreq.y.ltoreq.0.48,
0.ltoreq.z.ltoreq.0.48, and 0.08<y+z.ltoreq.0.48.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a Continuation Application of PCT Application No.
PCT/JP2004/004509, filed Mar. 30, 2004, which was published under
PCT Article 21(2) in Japanese.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an exhaust gas-purifying
catalyst, and in particular to a motor vehicle exhaust
gas-purifying catalyst containing a zeolite and a perovskite
composite oxide.
[0004] 2. Description of the Related Art
[0005] In recent years, the regulation of exhaust gas of motor
vehicles is being made stricter, increasing the necessity for
further decreasing the amounts of hydrocarbons (HC), carbon
monoxide (CO) and nitrogen oxides (NO.sub.x) in the exhaust gases.
Conventionally, a carried catalyst in which a noble metal (catalyst
active component) such as rhodium (Rh) or palladium (Pd) is carried
on a porous carrier has been widely employed as an exhaust
gas-purifying catalyst for motor vehicles. Such a carried catalyst
is called a three-way catalyst because it can oxidize CO and HC and
reduce NO.sub.x in the exhaust gas simultaneously.
[0006] The fuel is fed in a greater amount into a motor vehicle
engine during startup, leading to a smaller air-to-fuel ratio (A/F)
in the exhaust gas, i.e., a so-called rich (reductive) atmosphere,
resulting in increase in the amount of HC and CO in the exhaust
gas. In addition, the exhaust gas-purifying function is not
exhibited sufficiently during the engine startup at which the
catalyst is at low temperature, resulting in decrease particularly
in the HC-purifying efficiency.
[0007] Catalysts are known, which use a zeolite adsorbent that
adsorbs cold HC, in order to improve the purification efficiency of
the HC in cold exhaust gas emitted during motor vehicle engine
startup. For example, Jpn. Pat. Appln. KOKAI Publication No.
2-56247 discloses an exhaust gas-purifying catalyst having a first
catalyst layer containing a zeolite as a principal component and a
second catalyst layer formed thereon containing a noble metal
catalyst as a principal component. Jpn. Pat. Appln. KOKAI
Publication No. 7-96183 discloses an exhaust gas-purifying catalyst
of a structure in which an HC-adsorbing layer containing a zeolite
as a principal component and a porous HC-oxidizing layer containing
palladium oxide are laminated via a barrier layer. Further, Jpn.
Pat. Appln. KOKAI Publication No. 7-148429 discloses a catalyst
having an HC-adsorbing layer containing a zeolite and a catalyst
metal dispersion layer formed thereon.
[0008] On the other hand, Jpn. Pat. Appln. KOKAI Publication No.
62-282642 discloses a catalyst carrying palladium as a perovskite
composite oxide, as a catalyst that is suppressed in sintering of
palladium under a high-temperature reductive atmosphere to enhance
purification efficiency. Further, Jpn. Pat. Appln. KOKAI
Publication No. 3-131342 discloses a perovskite composite oxide
containing lanthanum aluminate added with platinum or
palladium.
[0009] However, these prior art catalysts are still low in
efficiency of purifying the HC in the cold motor vehicle exhaust
gas and difficult to maintain the catalytic activity over a long
period of time.
[0010] Thus, an object of the present invention is to provide an
exhaust gas-purifying catalyst that is superior in efficiency of
purifying the HC in the cold exhaust gas emitted during engine
startup of a motor vehicle and retains its HC-purifying activity
for a long period of time.
BRIEF SUMMARY OF THE INVENTION
[0011] The present inventors have made intensive studies in an
attempt to achieve the object above, and found that a catalyst
containing both a zeolite and a palladium-containing perovskite
composite oxide has a HC-purifying efficiency not foreseeable from
a catalyst containing a zeolite and palladium alone or a catalyst
consisting of a palladium-containing perovskite composite oxide
alone (synergistic effect).
[0012] Thus, according to the present invention there is provided
an exhaust gas-purifying catalyst comprising a zeolite and a
perovskite composite oxide having palladium placed at its B
site.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0013] FIG. 1 is a schematic sectional view illustrating a
structure of a catalyst according to a first embodiment of the
present invention; and
[0014] FIG. 2 is a schematic sectional view illustrating a
structure of a catalyst according to a second embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The present invention will be described in more detail
below.
[0016] An exhaust gas-purifying catalyst according to the present
invention comprises a zeolite and a perovskite composite oxide
containing palladium placed at its B site. The zeolite adsorbs cold
HC during start-up of a motor vehicle engine. The perovskite
composite oxide purifies the HC released from the zeolite while
hot.
[0017] The zeolite used in the present invention includes ZSM-5
zeolite, mordenite, ferrierite, Y zeolite, and .beta. zeolite.
Among them, use of .beta.-zeolite, which is superior in adsorption
of the HC in the exhaust gas, is preferable. The zeolite used
preferably has a silicon/aluminum (Si/Al) molar ratio of 300 to
900, from the viewpoint of the durability of the catalyst.
[0018] Generally, a perovskite composite oxide is represented by a
formula: ABO.sub.3, where A represents a cation located at the A
site, and B represents a cation located at the B site. In the
perovskite composite oxide containing palladium (Pd) at the B site
used in the present invention, such behavior is repeatedly
exhibited that Pd solid-solutions into the perovskite composite
oxide under an oxidative (lean) atmosphere and Pd precipitates onto
the surface of the perovskite composite oxide under a reductive
atmosphere, thus, prohibiting grain growth and deterioration in
purification efficiency over a long period of time.
[0019] The Pd-containing perovskite composite oxide used in the
present invention can be preferably represented by a general
formula (1):
(A.sup.1).sub.a(A.sup.2).sub.1-a(Pd).sub.b(B').sub.1-bO.sub.3 (1)
where each of a and b denotes the atomic proportions of the
respective elements, wherein 0<a.ltoreq.1, and 0<b<1.
A.sup.1 and optional A.sup.2 occupy the A site, while Pd and B'
occupy the B site.
[0020] In the formula (1), A.sup.1 represents at least one first
rare-earth element selected from the group consisting of rare earth
elements taking no valence other than trivalence. A.sup.2
represents at least one second rare-earth element selected from the
group consisting of rare-earth elements excluding the rare-earth
elements that can have a valence lower than trivalence. B'
represents at least one element selected from the group consisting
of transition elements excluding cobalt (Co), palladium (Pd) and
rare-earth elements (hereinafter, referred to as "specified
transition element(s)"), and aluminum (Al).
[0021] The perovskite composite oxide represented by the formula
(1) has a perovskite structure, and rare-earth elements are placed
at its A sites. More specifically, the first rare-earth element Al
taking no valence other than trivalence is always placed at the A
site, but no rare-earth element that can have a valence lower than
trivalence (for example, rare-earth element that can take both
bivalence and trivalence, such as Sm, Eu, Tm, or Yb) is placed
there. A transition element excluding Co, Pd and rare-earth
elements, i.e., the specified transition element, and/or Al are
placed together with Pd at the B site.
[0022] The first rare-earth element A.sup.1 necessarily placed at
the A site is an always trivalent rare-earth element. In other
words, the first rare-earth element A.sup.1 is a rare-earth element
excluding rare-earth elements that can take both trivalence and
quadrivalence such as cerium (Ce), praseodymium (Pr) and terbium
(Tb) and excluding rare-earth element that can take both bivalence
and trivalence such as samarium (Sm), europium (Eu), thulium (Tm)
and ytterbium (Yb). Examples of the rare-earth element A.sup.1
include scandium (Sc), yttrium (Y), lanthanum (La), neodymium (Nd),
promethium (Pm), gadolinium (Gd), dysprosium (Dy), holmium (Ho),
erbium (Er), and lutetium (Lu). These rare-earth elements may be
used alone or in combination.
[0023] Thus, in the perovskite composite oxide of the formula (1),
at the A site, the first rare-earth element A.sup.1 such as
scandium (Sc), yttrium (Y), lanthanum (La), neodymium (Nd),
promethium (Pm), gadolinium (Gd), dysprosium (Dy), holmium (Ho),
erbium (Er), or lutetium (Lu) is always placed, and the second
rare-earth element A.sup.2 that can take both trivalence and
quadrivalence such as cerium (Ce), praseodymium (Pr) or terbium
(Tb) is optionally placed.
[0024] By placing no rare-earth element that can have a valence
lower than trivalence but the first rare-earth element A.sup.1
taking no valence other than trivalence always and placing the
second rare-earth element A.sup.2 that can take both trivalence and
quadrivalence optionally at the A site in the perovskite composite
oxide of the formula (1), it become possible to make the Pd stably
present in the perovskite structure and accelerate the responses of
solid-solutioning of the Pd under an oxidative atmosphere and of
precipitation thereof under a reductive atmosphere.
[0025] In the formula (1), the atomic proportion of the first
rare-earth element A.sup.1 ("a" in the formula (1)) is preferably
0.6 to 1 (i.e., the atomic proportion of the second rare-earth
element A.sup.2 ("1-a" in the formula (1)) is 0.4 to 0), and more
preferably 0.8 to 1 (i.e., the atomic proportion of the second
rare-earth element A.sup.2 is 0.2 to 0). If the atomic proportion
of the first rare-earth element A.sup.1 at the A site is less than
0.6, stabilization of Pd in the perovskite structure may not be
effected in some cases.
[0026] As described above, the perovskite composite oxide of the
formula (1) includes those in which only the first rare-earth
element A.sup.1 (a=1 in the formula (1)) is placed at the A site,
and those in which both the first rare-earth element A.sup.1 and
the second rare-earth element A.sup.2 (0<a<1 in the formula
(1)) are placed at the A site. Of the two, the perovskite composite
oxide in which only the first rare-earth element A.sup.1 is placed
at the A site is preferable. By placing the first rare-earth
element A.sup.1 alone at the A site, it is possible to further
stabilize Pd in the perovskite structure.
[0027] Examples of the specified transition metal placed at the B
site together with Pd include, but are not limited to, elements in
the periodic table (IUPAC, 1990) having an atom number of 22 (Ti)
to 30 (Zn), 40 (Zr) to 48 (Cd), and 72 (Hf) to 80 (Hg) (excluding
Pd and Co). More specific examples of the transition metals include
titanium (Ti), chromium (Cr), manganese (Mn), iron (Fe), nickel
(Ni), copper (Cu), and the like. These specified transition
elements may be used alone or in combination of two or more.
[0028] Thus, a specified transition element such as titanium (Ti),
chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni), or copper
(Cu) and/or aluminum is placed together with palladium at the B
site.
[0029] In the perovskite composite oxide of the formula (1),
lanthanum, neodymium, yttrium, or a mixture thereof is preferable
as the first rare-earth element A.sup.1. Lanthanum, neodymium
and/or yttrium further stabilize the perovskite structure.
[0030] As the element placed at the B site together with palladium,
iron, manganese, aluminum, or a mixture thereof is preferable, with
iron being more preferable. Iron, manganese and/or aluminum further
stabilize the perovskite structure under a reductive atmosphere. In
particular, iron not only further stabilizes the perovskite
structure, but also can reduce environment load and enhance
safety.
[0031] In the perovskite composite oxide of the formula (1), an
atomic proportion of palladium placed at the B site ("b" in the
formula (1)) is preferably more than 0 and less than 0.5. If the
atomic proportion of palladium is 0.5 or more, palladium can not be
solid-solutioned into the composite oxide composition in some
cases, and the cost can not be lowered in some cases.
[0032] As apparent from the description above, the perovskite
composite oxide for use in the present invention is particularly
preferably a perovskite composite oxide represented by a formula
(1-1): APd.sub.pB.sub.1-pO.sub.3 (1-1) where A represents at least
one element selected from the group consisting of lanthanum,
neodymium and yttrium; B represents at least one element selected
from iron, manganese and aluminum; and p is 0<p<0.5.
[0033] Thus, in the perovskite composite oxide of the general
formula (1-1), lanthanum, neodymium and yttrium are placed,
respectively alone or in combination in arbitrary atomic
proportions at the A site. In addition, at the B site, palladium is
placed at an atomic proportion of more than 0 and less than 0.5,
preferably more than 0 and less than 0.2, and iron, manganese and
aluminum are placed respectively alone or in combination at
arbitrary atomic proportions such that the total amount thereof
corresponds to the balance of the palladium atomic proportion.
[0034] The catalyst according to the present invention may comprise
a zirconia-based composite oxide, in addition to the zeolite and
the Pd-containing perovskite composite oxide. The zirconia-based
composite oxide further stabilizes the Pd-containing perovskite
composite oxide and maintains the solid-solutioning/precipitation
behavior of Pd contained in the Pd-containing perovskite composite
oxide more stably for a long period of time. The zirconia-based
composite oxide may be present in a state mixed with the
Pd-containing perovskite composite oxide, in a state carrying the
Pd-containing perovskite composite oxide, and/or in a state mixed
in a catalyst layer containing a platinum group noble metal other
than Pd described below.
[0035] The zirconia-based composite oxide preferably contains, in
addition to zirconium, cerium and, optionally, lanthanum and/or an
Ln (at least one element selected from the group consisting of
neodymium (Nd), praseodymium (Pr) and yttrium (Y)). The
zirconia-based composite oxide can be represented by the following
formula: Zr.sub.1-(x+y+z)Ce.sub.xLa.sub.yLn.sub.zO.sub.2 (2) In the
formula (2), x, y and z represent atomic proportions of the
respective elements and satisfy the following relationships:
[0036] 0.2<x+y+z.ltoreq.0.6,
[0037] 0.12.ltoreq.x.ltoreq.0.5,
[0038] 0.ltoreq.y.ltoreq.0.48,
[0039] 0.ltoreq.z.ltoreq.0.48, and
[0040] 0.08<y+z.ltoreq.0.48.
[0041] In addition, a catalyst according to the present invention
preferably contains a platinum group noble metal other than Pd. A
catalyst containing such a platinum group noble metal can purify
the HC, NOx and CO in an exhaust gas more efficiently. Examples of
the platinum group noble metal include platinum, rhodium, and the
like; and these noble metals may be used alone or in combination.
The platinum group noble metal catalyst may be used in a state
supported on a carrier such as alumina. Also the zirconia-based
composite oxide may be supported on a carrier together with a
platinum group noble metal catalyst.
[0042] A catalyst according to the present invention is usually
supported as a layer on a heat-resistant support, in particular on
a monolithic support. Examples of the heat-resistant support
include monolithic supports having, in their axial direction,
tubular passages in which an exhaust gas flows, such as monolithic
honeycomb supports having a plurality of such tubular passages.
Such a heat-resistant support can be formed of a heat resistance
ceramic material such as cordierite.
[0043] A catalyst according to the present invention preferably
contains a perovskite composite oxide according to the present
invention in an amount of 0.1 to 1 part by weight based on 1 part
by weight of zeolite. If the amount of the perovskite composite
oxide is less than 0.1 parts by weight, the catalytic performance
may become insufficient in some cases. If the amount is more than 1
part by weight, the total coating amount may become too large and
hence the ignition characteristics may be lowered in some cases.
The perovskite composite oxide is more preferably used in an amount
of 0.1 to 0.5 parts by weight based on 1 part by weight of
zeolite.
[0044] When used together with the perovskite composite oxide in
the same layer, the zirconia-based composite oxide is preferably
used in an amount of 0.3 to 100 parts by weight with respect to 1
part by weight of the perovskite composite oxide according to the
present invention. If the total amount of the zirconia-based
composite oxide is less than 0.3 parts by weight, an effect of
dispersing the palladium-containing perovskite composite oxide may
become insufficient in some cases. If the total amount is more than
100 parts by weight, it may be uneconomical in some cases. The
zirconia-based composite oxide is more preferably used in a total
amount of 1 to 10 parts by weight with respect to 1 part by weight
of the perovskite composite oxide.
[0045] Further, the platinum group noble metal other than Pd, when
used, is preferably used in an amount of 0.1 to 10 g per liter of
the catalyst (0.1 to 10 g/L-cat).
[0046] The perovskite composite oxide according to the present
invention can be prepared by various methods known per se as
producing composite oxides, such as a coprecipitation method, an
alkoxide method, and a citrate complex method.
[0047] In order to prepare a perovskite composite oxide according
to the present invention by the coprecipitation method, an aqueous
solution can be prepared, which contains salts (starting metal
salts) of the metal elements constituting the perovskite composite
oxide (for example, A.sup.1, Pd, B' and optional A.sup.2 in the
formula (1)), and the solution can be added with an aqueous
alkaline solution or aqueous organic acid solution to coprecipitate
a salt containing the metals constituting the perovskite composite
oxide. The starting metals salt used in the coprecipitation method
include inorganic salts such as sulfates, nitrates, hydrochlorides,
and phosphates; and organic salts such as acetates and oxalates.
Preferable salts are nitrate salts. Examples of the aqueous
alkaline solution include an aqueous solution of a salt of an
alkali metal such as sodium or potassium, an aqueous solution of
ammonia or ammonium carbonate, and a known buffer solution. When an
aqueous alkaline solution is used, the solution is preferably added
such that a solution obtained after the addition of the aqueous
metal salt solution exhibits a pH of approximately 8 to 11.
Examples of the aqueous organic acid solution include an aqueous
solution of oxalic acid, citric acid, or the like.
[0048] The coprecipitate obtained may be filtered, washed, dried
preferably at 50 to 200.degree. C. for 1 to 48 hours, and baked at
400 to 1000.degree. C., preferably 650 to 1000.degree. C., for 1 to
12 hours, preferably 2 to 4 hours. Thus, a Pd-containing perovskite
composite oxide containing the metals at a ratio substantially the
same as the mixing ratio of the starting metal salts can be
obtained.
[0049] In order to prepare a perovskite composite oxide according
to the present invention by the alkoxide method, first, a metal
alkoxide solution is prepared by dissolving alkoxides of the metal
elements constituting the perovskite composite oxide (starting
metal alkoxides) in an organic solvent, and the metal alkoxide
solution can be added to deionized water to coprecipitate or
hydrolyze the metal alkoxides to produce a coprecipitate or
hydrolysate.
[0050] Examples of the alkoxy forming the alkoxide include an
alkoxy such as methoxy, ethoxy, propoxy, isopropoxy, or butoxy; an
alkoxyalcoholate such as methoxyethylate, methoxypropylate,
methoxybutyrate, ethoxyethylate, ethoxypropylate, propoxyethylate,
or butoxyethylate; and the like. Examples of the organic solvent
include aromatic hydrocarbons, aliphatic hydrocarbons, alcohols,
ketones, esters, or the like. Preferably, aromatic hydrocarbons
such as benzene, toluene, or xylene are mentioned.
[0051] The coprecipitate or hydrolysate obtained can be filtered,
washed, dried, and baked, as in the coprecipitation method. Thus, a
perovskite composite oxide containing the metals at a ratio
substantially the same as the mixing ratio of the starting metal
alkoxide can be obtained.
[0052] In order to prepare a Pd-containing perovskite composite
oxide by the citrate complex method, an aqueous solution containing
the citrate salts of the metal elements constituting the
Pd-containing perovskite composite oxide (starting metal citrate
salts) can be evaporated to dryness and the dried material can be
dried and baked as in the coprecipitation method. Thus, a
Pd-containing perovskite composite oxide containing metals at a
ratio substantially the same as the mixing ratio of the starting
metal citrate salts can be obtained.
[0053] The zirconia-based composite oxide for use in the present
invention may be prepared by a coprecipitation method, an alkoxide
method, or a slurry method, known per se.
[0054] In order to prepare the zirconia-based composite oxide by
the coprecipitation method, salts of the metal elements
constituting the zirconia-based composite oxide are used as the
starting metal salts in the production of the Pd-containing
perovskite composite oxide by the coprecipitation method. In order
to prepare the zirconia-based composite oxide by the alkoxide
method, alkoxides of the metal elements constituting the
zirconia-based composite oxide are used as the starting metal
alkoxides in the production of the perovskite composite oxide by
the alkoxide method.
[0055] In order to prepare the zirconia-based composite oxide by
the slurry method, a slurry is prepared by adding water to cerium
oxide powder in an amount of 10 to 50 times the weight of the
latter, and an aqueous solution containing zirconium salt and
optionally salts of lanthanum and Ln (for example, containing water
in an amount of 0.1 to 10 times the total weight of the metal salt)
is added to the slurry, which is sufficiently stirred, and dried
under a reduced pressure, and then dried and baked as in the
coprecipitation method.
[0056] The cerium oxide powder for use in the slurry method may be
a commercial product, but it preferably has a large specific
surface area for improvement in its oxygen storage capacity. For
example, the cerium oxide powder can have a specific surface area
of 40 to 100 cm.sup.2/g. The inorganic and organic acid salts used
in the coprecipitation method may be used as the zirconium,
lanthanum and Ln salts.
[0057] In the baking for preparing the zirconia-based composite
oxide, it is preferable that at least a part of the zirconia-based
composite oxide forms a solid solution to improve the heat
resistance of the zirconia-based composite oxide. The baking
conditions suitable for forming the solid solution can be properly
determined according to the composition of the zirconia-based
composite oxide and the ratio of the components.
[0058] In order to carry the perovskite composite oxide of the
invention on the zirconia-based composite oxide, though not
particularly limited, but for example, a zirconia-based composite
oxide may be added at the ratio noted above during the production
of the perovskite composite oxide.
[0059] More specifically, when the perovskite composite oxide
according to the present invention is produced by the
coprecipitation method, powder of the zirconia-based composite
oxide powder is added to the aqueous solution of the starting metal
salts prepared, the resultant mixture is added to the aqueous
alkaline solution or organic acid solution to effect
coprecipitation, and the obtained coprecipitate can be dried and
baked as described above.
[0060] When the perovskite composite oxide according to the present
invention is prepared by the alkoxide method, powder of the
zirconia-based composite oxide is added to the starting metal
alkoxide solution prepared, to effect precipitation by hydrolysis,
and the obtained precipitates can be dried and baked, as described
above.
[0061] When the perovskite composite oxide according to the present
invention is prepared by the citrate complex method, powder of the
zirconia-based composite oxide is added to the aqueous starting
metal citrate salt solution prepared and thereafter the aqueous
mixed citrate salt solution can be dried to dryness and baked as
described above.
[0062] The platinum group noble metal catalyst other than palladium
is usually used in a state carried on heat-resistant porous carrier
particles such as alumina. The carried platinum group noble metal
catalyst can be formed by coating an aqueous slurry containing the
refractory porous carrier particles and an aqueous solution of a
water-soluble noble metal compound such as platinum nitrate or
rhodium nitrate, which is dried at a temperature of 200 to
250.degree. C. for about 1 hour, and baked at 450 to 600.degree. C.
for about 2 hours. When the zirconia-based composite oxide is
caused to be co-present in the noble metal catalyst, powder of the
zirconia-based composite oxide may be added to the slurry noted
above.
[0063] According to a first embodiment of the present invention,
the exhaust gas-purifying catalyst according to the present
invention has one layer in which the Pd-containing perovskite
composite oxide and the zeolite are mixed together, on a
heat-resistant support. FIG. 1 is a schematic sectional view
illustrating a catalyst according to the first embodiment. A layer
11 in which a perovskite composite oxide according to the present
invention and a zeolite are mixed is formed on a heat-resistant
support 10. The layer 11 may contain the zirconia-based composite
oxide additionally. The zirconia-based composite oxide may be
simply contained in the layer 11 or may carry the Pd-containing
perovskite composite oxide. It is preferable that a second layer 12
containing a platinum group noble metal other than Pd is formed on
the layer 11. A zirconia-based composite oxide may be incorporated
into the second layer 12.
[0064] The layer 11 of a mixture containing the perovskite
composite oxide and a zeolite according to the present invention
can be formed by preparing an aqueous slurry by using powder of the
perovskite composite oxide according to the present invention and a
zeolite together with a binder such as acidic alumina sol, coating
the slurry on the surface of the support 10, and drying and then
baking the coated slurry at 500.degree. C. to 600.degree. C. for 1
to 2 hours. In this case, when a zirconia-based composite oxide is
incorporated into the aqueous slurry, a layer 11 containing the
zirconia-based composite oxide together with the powder of the
perovskite composite oxide according to the present invention and
zeolite powder can be obtained.
[0065] According to a second embodiment of the present invention,
the exhaust gas-purifying catalyst according to the present
invention comprises the perovskite composite oxide according to the
present invention and a zeolite as separate layers on the
heat-resistance support. Usually, the zeolite-containing layer is
formed closer to the support than the perovskite composite
oxide.
[0066] FIG. 2 is a schematic sectional view illustrating a catalyst
according to the second embodiment. On a heat-resistant support 10,
a first layer 21 containing a zeolite is formed, and a second layer
22 containing a Pd-containing perovskite composite oxide is formed
thereon. A zirconia-based composite oxide may be incorporated into
the second layer 22. It is preferable that a third layer 23
containing a platinum group noble metal other than Pd is formed on
the second layer 22. A zirconia-based composite oxide may also be
incorporated into the third layer.
[0067] The first layer containing a zeolite can be formed by
preparing an aqueous slurry by using a zeolite powder, a binder
such as acidic silica sol and water, coating the aqueous slurry on
the surface of the support 10, and drying and then baking the
coated slurry at 500.degree. C. to 600.degree. C. for 1 hours to 2
hours.
[0068] The second layer 22 containing the perovskite composite
oxide can be formed by preparing an aqueous slurry by using powder
of the perovskite composite oxide, a binder such as acidic alumina
sol and water, coating the slurry on the surface of the support 10,
and drying and then baking the coated slurry at 500.degree. C. to
600.degree. C. for 1 to 2 hours. In this case, powder of the
zirconia-based composite oxide may be incorporated into the aqueous
slurry.
[0069] In the catalyst according to the present invention, HC in
the initial cold exhaust gas emitted from a motor vehicle during
engine startup is adsorbed on the zeolite, and the Pd in the
perovskite composite oxide purifies the HC released from the
zeolite during hot. In the perovskite composite oxide, such
behavior is repeatedly exhibited for a long period of time that Pd
solid solutions into the perovskite composite oxide under an
oxidative (lean) atmosphere and Pd precipitates onto the surface of
the perovskite composite oxide under a reductive atmosphere,
suppressing growth of the particles. Thus, the catalyst according
to the present invention is superior in the efficiency of purifying
the HC in the cold exhaust gas emitted from a motor vehicle during
engine startup and retains its HC-purifying activity for a long
period of time.
[0070] The present invention will be described below by way of
specific Examples. However, the present invention should not be
restricted these Examples.
PREPARATION EXAMPLE 1
Preparation of Pd-Containing Perovskite Composite Oxide
[0071] 40.6 g (0.100 moles) of lanthanum ethoxyethylate and 30.7 g
(0.095 moles) of iron ethoxyethylate were placed in a
round-bottomed flask having a capacity of 500 mL, added with 200 mL
of toluene, and dissolved by stirring, to give a mixed alkoxide
solution. Then, a solution prepared by dissolving 1.52 g (0.005
moles) of palladium acetylacetonate in 100 mL of toluene was added
to the mixed alkoxide solution above, to give a homogeneous mixture
solution containing La, Fe, and Pd.
[0072] Then, to this solution, 200 mL of deionized water was added
dropwise over approximately 15 minutes. Then, a brown viscous
precipitate was formed by hydrolysis.
[0073] Then, stirring at room temperature for 2 hours was
conducted, and the toluene and water were stripped off under
reduced pressure, to provide a precursor of a La--Fe--Pd composite
oxide. The precursor was placed in a petri dish, air-dried at
60.degree. C. for 24 hours, and heat-treated in the air using an
electric furnace at 650.degree. C. for 2 hours, to give a powder of
a Pd-containing perovskite composite oxide of
La.sub.1.0Fe.sub.0.95Pd.sub.0.05O.sub.3.
PREPARATION EXAMPLE 2
Preparation of Zirconia-Based Composite Oxide
[0074] 25.6 g (0.076 moles) of zirconium oxychloride, 7.8 g (0.018
moles) of cerium nitrate, 1.7 g (0.002 moles) of lanthanum nitrate
and 1.8 g (0.004 moles) of neodymium nitrate were dissolved in 100
mL of deionized water, to give a mixed aqueous solution. The mixed
aqueous solution was added gradually dropwise to an aqueous
alkaline solution prepared by dissolving 25.0 g of sodium carbonate
in 200 mL of deionized water, giving a coprecipitate. The
coprecipitate was washed thoroughly with water and filtered, and
then dried sufficiently at 80.degree. C. in vacuo. Thereafter, the
coprecipitate was heat-treated (calcined) at 800.degree. C. for 1
hour, to give a powder of a zirconia-based composite oxide into
which the cerium and lanthanum were solid-solutioned, consisting of
Zr.sub.0.76Ce.sub.0.18La.sub.0.02Nd.sub.0.04O.sub.2.
EXAMPLE 1
[0075] A slurry I consisting of 15 g of the Pd-containing
perovskite composite oxide powder of Preparation Example 1, 15 g of
acidic alumina sol, 100 g of .beta.-zeolite powder and 150 g of
water was coated on a monolithic honeycomb cordierite support
having a capacity of 1 L, dried at 250.degree. C. for 1 hour, and
baked at 500.degree. C. for 1 hour, to give a catalyst A.
EXAMPLE 2
[0076] A slurry II consisting of 100 g of .beta.-zeolite powder, 30
g of acidic silica sol and 150 g of water was coated on a
monolithic cordierite support having a capacity of 2 L and dried at
250.degree. C. for 1 hour, to form a zeolite layer on the support.
Then, a slurry III consisting of 15 g of the Pd-containing
perovskite composite oxide powder of Preparation Example 1, 10 g of
acidic alumina sol and 20 g of water was coated on surface of the
zeolite layer. Thereafter, the slurry III was dried at 250.degree.
C. for 1 hour and baked at 500.degree. C. for 1 hour, to give a
catalyst B.
EXAMPLE 3
[0077] The slurry II prepared in Example 2 was coated on a
monolithic honeycomb cordierite support having a capacity of 1 L
and baked at 250.degree. C. for 1 hour, to form a zeolite layer on
the support. Then, a slurry IV consisting of 60 g of the
zirconia-based composite oxide powder of Preparation Example 2
carrying 15 g of the Pd-containing perovskite composite oxide
powder of Preparation Example 1, 30 g of acidic alumina sol and 100
g of water was coated on the surface of the zeolite layer. The
slurry IV was then dried at 250.degree. C. for 1 hour and baked at
500.degree. C. for 1 hour, to give a catalyst C.
EXAMPLE 4
[0078] A slurry V consisting of an aqueous platinum nitrate
solution (containing 0.5 g of platinum), an aqueous rhodium nitrate
solution (containing 0.25 g of rhodium), 70 g of alumina powder and
80 g of water was coated on the surface of the catalyst C obtained
in Example 3, dried at 250.degree. C. for 1 hour and then baked at
500.degree. C. for 1 hour, to give a catalyst D.
EXAMPLE 5
[0079] A slurry VI consisting of an aqueous platinum nitrate
solution (containing 0.5 g of platinum), an aqueous rhodium nitrate
solution (containing 0.25 g of rhodium), 40 g of alumina powder, 30
g of the zirconia-based composite oxide powder of Preparation
Example 2 and 80 g of water was coated on the surface of the
catalyst C obtained in Example 3, dried at 250.degree. C. for 1
hour and then baked at 500.degree. C. for 1 hour, to give a
catalyst E.
Comparative Example 1
[0080] A slurry VII consisting of 10 g of the perovskite composite
oxide powder of Preparation Example 1, 10 g of acidic alumina sol
and 10 g of water was coated on a monolithic honeycomb cordierite
support having a capacity of 1 L, dried at 250.degree. C. for 1
hour and baked at 500.degree. C. for 1 hour, to give a catalyst
F.
Comparative Example 2
[0081] The slurry II prepared in Example 2 was coated on a
monolithic support having a capacity of 1 L and dried at
250.degree. C. for 1 hour, to form a zeolite layer on the support.
Then, a slurry VIII consisting of an aqueous palladium nitrate
solution (containing 0.3 g of palladium), 30 g of acidic alumina
sol and 100 g of water was coated on the surface of the above
zeolite layer. Thereafter, the slurry VIII was dried at 250.degree.
C. for 1 hour and baked at 500.degree. C. for 1 hour, to give a
catalyst G.
Comparative Example 3
[0082] The slurry V prepared in Example 4 was coated on the surface
of the catalyst F prepared in Comparative Example 1, dried at
250.degree. C. for 1 hour and baked at 500.degree. C. for 1 hour,
to give a catalyst H.
Comparative Example 4
[0083] The slurry V prepared in Example 4 was coated on the surface
of the catalyst G prepared in Comparative Example 2, dried at
250.degree. C. for 1 hour and baked at 500.degree. C. for 1 hour,
to give a catalyst I.
<Evaluation of Exhaust Gas-Purifying Capacity>
[0084] Each of the catalysts A to I prepared in Examples 1 to 9 was
mounted on a gasoline engine having a displacement of 4 L, and
subjected to an endurance test over 100 hours under the conditions
of an inflow gas air fuel ratio of 14.6 and a catalyst inflow gas
temperature of 900.degree. C., while a lead-free gasoline was
supplied to the engine.
[0085] Each of the catalysts A to I was then installed at a site 30
cm directly below the engine of an actual vehicle (displacement:
2.2 L). The engine of the actual vehicle was operated in the U.S.
LA #4 mode, the exhaust gas emitted from the engine was purified,
and the purification rate of non-methane hydrocarbons (NMHC) in the
exhaust gas passing the catalyst was measured. The purification
rate was measured by using a motor vehicle exhaust-gas analyzer,
while the exhaust gas from the tail pipe was collected by using a
chassis dynamo tester. Results are shown in Table 1. TABLE-US-00001
TABLE 1 Pt + Pd + Pd-carrying catalyst Rh-carrying catalyst NMHC
NMHC Purification Purification Example No. rate (%) Example No.
rate (%) Example 1 90.2 Example 4 96.4 (Catalyst A) (Catalyst D)
Example 2 93.5 Example 5 98.2 (Catalyst B) (Catalyst E) Example 3
95.3 Comparative 84.5 (Catalyst C) Example 3 (catalyst H)
Comparative 83.0 Comparative 90.1 Example 1 Example 4 (catalyst F)
(catalyst I) Comparative 85.5 Example 2 (catalyst G)
[0086] Comparison of the results of the purification rate of
Comparative Examples 1 to 2 with those of the catalysts of Examples
1 to 3 reveals that, although all of the catalysts contains Pd as a
noble metal, the catalyst F of Comparative Example 1 consisting of
the Pd-containing perovskite composite oxide alone and the catalyst
G of Comparative Example 2 containing Pd and zeolite have NMHC
purification rates of 83.0 and 85.5%, and by contrast, the
catalysts A to C of Examples 1 to 3 containing the Pd-containing
perovskite composite oxide and zeolite have NMHC purification rates
all of more than 90%, clearly indicating the synergistic effect of
the combined use of a Pd-containing perovskite composite oxide and
zeolite. The synergistic effect is also obvious from the comparison
of the results of the NMHC purification rate of the catalysts H and
I of Comparative Examples 3 and 4 with those of the catalysts D and
E of Examples 4 and 5.
[0087] Further, comparison of the results of the NMHC purification
rate of the catalyst A of Example 1 with those of the catalyst B of
Example 2 reveals that forming the Pd-containing perovskite
composite oxide and zeolite into separate layers is more preferable
than mixing them in a single layer.
[0088] Moreover, comparison of the results of the NMHC purification
rate of the catalysts A and B of Examples 1 to 2 with those of the
catalyst C of Example 3, reveals that it is preferable that the
Pd-containing perovskite composite oxide is co-present together
with the zirconia-based composite oxide.
[0089] Further, comparison of the results of the NMHC purification
rate of the catalysts A to C of Examples 1 to 3 with those of the
catalysts D and E of Examples 4 and 5 reveals that it is preferable
for the catalyst preferably to contain, in addition to Pd, the
other platinum group metal.
* * * * *